Normal Physiology Small Bowel and Colon Flashcards
What is the mode of primary motility in the small bowel?
segmentation in the fed state –> peristaltic graident via pacemaker cells 9-12 contractions/min –> transit 3-5 hours
What is the gastroileal reflex?
gastrin activates segmentation in ileum
What is the intrinsic reflex arc?
mechanoreceptors activate motility in small bowel
What is the migrating motor complex?
occurs during fasting state –> sequential organized short waves initiated by motilin in ileum –> accompanied by pyloric sphincter relaxation
What initiates the migrating motor complex?
motilin in ileum
What turns off the migrating motor complex?
gastrin secretion (meal time)
Where is the vomiting center?
medulla: lateral reticular formation
What are the vomiting afferents?
irritant receptors in gut and other organs
What are the vomiting efferents?
vagal, phrenic, spinal, S, P/S
Mechanism of vomiting
saliva –> upper intestine contraction –> pylorus contraction –> abdominal muscles/diaphragm contraction –> LES/esophagal dilation –> glottis closes
Does the small bowel secrete enzymes?
no –> water and mucus for hydrolysis and lubrication (driven by Cl- secretion)
What drives small bowel secretion?
Cl- secretion in crypts
What regulates small bowel secretion?
cAMP
What is the primary channel involved in small bowel secretion?
CFTR
How is small bowel absorption regulated?
it is not –> absorbs everything isotonically and doesn’t concentrate the stuff unlike the kidney
2 types of surface epithelial cells in small bowel
villus and crypt cells of lieberkuhn
What is the function of villi?
absorption
What is the function of crypts?
secretion of Cl-
What is the average lifespan of intestinal epithelia?
3 days
What enzyme initiates digestion of carbohydrates?
salivary amylase
What carbohydrate digestion occurs in small bowel?
pancreatic amylase secretion and degradation of complex carbs to disaccharides –> final step = brush border disaccharidases
How are sugars transported in the small bowel?
cotransport with Na+
What enzyme initiates digestion of proteins?
HCl-pepsin in stomach
What protein digestion occurs in small bowel?
pancreatic proenzymes activated in lumen –> enterokinase catalyzes trypsinogen –> protease activation –> final step = brush border aminopeptidases
How are proteins transported in the small bowel?
- cotransport with Na+ w/ carriers for neutral basic and acid AAs
- cotransport of di/tripeptides with H+ and broken down into AAs in enterocyte
What happens to fat in the lumen of the small bowel?
emulsification/digestion
How is lipid absorbed by the small bowel?
micell transfer across apical cell membrane
What happens to lipids in enterocytes?
processing and reconstitution into bigger lipid molecules for lymphatic/bloodstream export
What lipodigestive mechanisms take place in the stomach?
- gastric lipase breaks down TG –> FA and DG
2. DG and phospholipids stabilize emulsion
What lipodigestive mechanisms take place in the duodenum?
- bile salts stabilize emulsion
2. pancreatic lipase converts TG –> FA + MG
What is inside a stable duodenal emulsion?
tg, dg, cholesterol, fat soluble vitamines
What is outside a stable duodenal emulsion?
Phospholipids, MG, FA, bile salts –> hydrophilic
What is colipase and where is it secreted?
pancreas –> brings lipase and TG together –> 50% decrease in lipase function without colipase
What is phospholipase A2 and where is it secreted?
pancreas –> digests phospholipids on emulsion surface, helps anchor lipase to emulsion, activated by bile salts
What activates phospholipase A2
bile salts
Do brushborder enterocytes digest fat?
no
What is the fate of bile salts?
some excreted but much of it eventually reabsorbed in terminal ileum
How are fats packaged for transport to lymphatics?
chylomicrons coated with ApoA,B –>exocytic release
How are vitamins digested?
they are not –> absorbed whole
What are the fat soluble vitamins?
ADEK
How is B12 absorbed?
IF from gastric parietal cells transport B12 from proximal bowel to ileum where IF-B12 complex is absorbed via transport protein at terminal ileum
How are water soluble vitamins absorbed?
B, C via simple diffusion or carrier mediated transport
Where does B12 bind intrinsic factor?
in proximal small bowel after bieng freed from carrier R protein
Relative rate of absorption of salt/water in three parts of small bowel?
ileum > jejunum > duodenum
How is salt/water absorbed in small bowel?
actively absorbed via Na+/K+ Atpase on basolateral membrane
Is there more Na in the: lumen, cell, blood
blood
Is there more K in the: lumen, cell, blood
cell
What is the voltage of the lumen
-4mv
What is the voltage of an enterocyte?
-60mv
What is the voltage of blood next to an enterocyte?
0mv
Functions of colon
salt and water absorption (no contentration), waste storage, no digestion
<p>
| What is more permeable to ions? small/large bowel</p>
<p>
| small bowel</p>
What ions are absorbed in colon?
Na/Cl/water
What ions are secreted in colon?
K+ (unlike small bowel), HCO3
What is more permeable to water? small/large bowel
colon is slower and absorbs less
Small bowel or colon? digestion
SB
Small bowel or colon? na/nutrient transport
SB
Small bowel or colon? K secretion
colon
Small bowel or colon? net hco3 secretion
large bowel
Small bowel or colon? water absorption
sb > colon
Small bowel or colon? concentration
neither
What regulates segmentation?
ANS
What is the gastrocolic reflex?
simultaneous strong contractions in proximal colon –> mass movement response to gastrin and extrinsic nerves
What is secreted by bacteria that nourishes colon?
SCFA (short chain fatty acid), gas, vitamin K, stimulate immunity/IgA secretion
T/F defecation involves both voluntary and involuntary muscles
T
